As I read it, a Brown Dwarf is not a very energetic star, and so does not expel fuel mass very far or fast, so a fuel scoop cannot collect any. However, other stars burn hot enough to expel fuel mass far enough to be useable (while not so hot as to cook a ship), while other stars burn so hot as to be unapproachable to scoop.
Think of them as fuel station pumps. Some will take an hour to dispense a litre of fuel, others take 10 seconds, while some will spray fuel at the rate of 100 gallons a second. I would go to the middle option!
Feel free to take this with a pinch of salt. It is just how I understand it, but I could easily be wrong!